Non-Destructive Testing (NDT) technology has extensive scientific foundation and application fields, which is an important measure to guarantee the quality of products and infrastructures. Eddy Current Pulsed Thermography (ECPT) combines the advantages of eddy current and infrared thermal imaging, therefore it can provide non-contact, real-time NDT for a large area of defects of various depths, and wildly used in the field of conductive material's Non-Destructive Testing, and has become a major method for analyzing the defect of conductive material.
In ECPT, when an electromagnetic (EM) field is applied to a conductive material, the temperature increases due to resistive heating caused by induced electric current, which is also known as Joule heating. When a defect, for example a crack exists in the conductive material, eddy current (EC) behavior will be forced to divert, which leads to an increase in EC density in the vicinity of the crack, especially, at the tip and bottom area of the crack, the EC density is much greater than that of other area, which directly leads to spatial variation in Joule heating, and hot spots can be found around the tip and bottom area of the crack. This phenomenon mentioned above can be used to detect the defect of a conductive material. Moreover, the heating spatial and temporal distribution on resultant surface can be captured and recorded by IR camera. After post-processing, the defect can be quantitatively detected and analyzed.
At present, an enormous amount of achievement has been made in the detection and characterization of conductive material's defect, for example, Independent Component Analysis (ICA). In ICA, a thermogram sequence captured and recorded by IR camera is processed by independent component analysis, and according to the spatial and temporal distribution in the thermogram sequence, the defect information can be enhanced and extracted under no prior information. ICA is the prior art, more specific steps can be found in the paper “A. Hyvarinen, J. Karhunen, and E. Oja, “Independent component analysis and blind source separation,” John Wiley & Sons, pp. 20-60, 2001”.
As a further research in ICA, a new method for analyzing the defect of conductive material has been put forward in the paper “Bin Gao, Libing Bai, Guiyun Tian, W. L. Woo and Yuhua Cheng, “Automatic Defect Identification of Eddy Current Pulsed Thermography Using Single Channel Blind Source Separation,” IEEE Transactions on Instrumentation and Measurement, vol. 63, no. 4, pp. 913-922, 2014”. In the new method, thermogram sequence is pre-processed by conventional ICA, and then, the independent component with maximum kurtosis is selected and reconstructed as a defect image matrix according to the size of the original thermogram.
However, in prior art, the study of quantitative detection of defect is inadequate and lack of a appropriate automatic segmentation method of extraction, identification and quantification of defect, which obstructs the application of ECPT to NDT industry.